spill the beans

spill the beans  {v. phr.},  {informal}

Раскрывать секрет тому, кто не должен о нем знать.

John's friends were going to have a surprise party for him, but Tom spilled the beans.

Ср.:

GIVE AWAY (3),

LET THE CAT OUT OF THE BAG,

TELL TALES OUT OF SCHOOL.